A bi-metallic thermostat for a single stage heating system usually includes a wire wound resistive element called an anticipator. The anticipator is heated by current that flows through the resistor when the zone heating device is energized. Its purpose is to cause the bi-metalic strip actuated switch to de-energize the zone heating system sooner than it otherwise would under the influence of zone temperature, thereby minimizing setpoint overshoot.
In a multi-stage temperature conditioning system, a conventional thermostat control might comprise a plurality of such single stage bi-metallic switches, with a predetermined mechanical offset between the setpoint temperatures at which each stage is energized by the control. Controls for multi-stage systems with heating and cooling capability are even more complex, often with separate anticipation means for each stage in the heating and cooling modes of operation. As an alternative to these electromechanical multi-stage devices, solid-state controls incorporating both analog and digital electronic circuits have been developed, that offer greater control flexibility. Particularly, in microprocessor based controls, parameters such as hysteresis and dead-band may be implemented in software and easily modified for adaptation of the control to a specific installation. The control can be programmed to respond to variables determined by the characteristics of the multi-stage system and the building in which it is installed. One such variable is the rate of change of the zone temperature. Examples of multi-stage controls using the rate of change of zone temperature are described in U.S. Pat. Nos. 4,265,299 and 4,338,791.
The '299 patent discloses an analog control for a heat pump system that initiates supplemental electric heat following a setpoint increase only if the rate of zone temperature change is below a predetermined value. This avoids the use of relatively expensive electric resistance heating to recover from a setback temperature if the system can reach setpoint in a reasonable time using only the heat pump capacity. A more comprehensive control is disclosed in the '791 patent. It uses a microprocessor programmed with user inserted temperature setpoints, and responds to the actual room temperature and rate of change of room temperature in controlling the operating state of the system. A higher energy consumption operating state is enabled only when a periodic check of the rate of zone temperature change indicates that the desired temperature will not be achieved in a selected time period; however, rate of change is not used to determine if a lower energy consumption stage is desirable, for purposes of de-energizing one of the stages. Tracking the rate of zone temperature change allows the control to reach a setpoint temperature in a desired time interval, with minimal overshoot. The control thus does not require an anticipation means.
The control scheme of the '791 patent changes to a lower energy consumption operating stage in accordance with logic relating the deviation from setpoint to a specific predetermined change of state. In addition, the control is designed for use in a system with a specific number of operating states, with specific numbered states being associated with predetermined operating parameters, such as rate of zone temperature change and deviation of the zone temperature from setpoint. The control is thus not general in application, i.e., equally usable with systems having different numbers of operating states.
In consideration thereof, it is an object of this invention to provide a control for a multi-stage temperature conditioning that is general in application to different systems comprising various numbers and kinds of temperature conditioning stages.
A further object is to eliminate the need for anticipation means in the control, while minimizing overshoot of the zone setpoint.
A still further object of the invention is to selectively energize and de-energize stages of temperature conditioning as a function of the rate of change of zone temperature.
These and other objects of the invention will be apparent from the description of the preferred embodiment which follows hereinbelow and the attached drawings.